In the construction of long retracting soot blowers of the longer travels required by the increasing size of large public utility boilers it has become necessary to extend the lance tube in cantilever fashion into the boiler for distances of forty to fifty or fifty-five feet. The forces resulting not only from the weight of the lance tube and carriage but from the overhung situation which exists when the lance tube is projected creates severe problems due to the high stresses applied to all of the supporting and guiding components. A commercially successful soot blower designed for travels of as much as forty-five feet is disclosed in U.S. Pat. No. 3,439,376, granted Apr. 22, 1969 to John E. Nelson and John R. Saltz. In the Nelson-Saltz construction the carriage to which the lance tube is connected and by which it is actuated is supported and guided by and between the internal walls of a box beam by means of rollers each of which travels between a pair of upper and lower tracks which are spaced apart a distance conforming to the diameter of the rollers. When the lance tube is retracted far enough so that the system is not overbalanced by the portion of the lance tube remaining in the boiler, the rollers bear against the bottom tracks, while when the load is overbalanced, the rollers bear against and roll along the undersides of the upper tracks. It will be recognized that the resultant vertical forces tend to separate the tracks and impose loadings which tend to spread the tracks and the walls of the box beam. It will be appreciated also by those familiar with the construction and operation of soot blowers that when a long lance is projected into the boiler, great power is required to drive it longitudinally, particularly in the retracting direction since the pressure of the blowing medium opposes retraction and adds to the unavoidably high frictional resistance. Because of the angulation or curvature of rack and pinion gear teeth, the forces generated therebetween in driving the lance do not act along a line parallel to the direction of movement of the lance but rather at some angle relative to the desired direction of movement. Thus, these driving forces produce movement generating force components acting parallel to the direction of movement and cam action components acting in a direction perpendicular to the direction of movement. These cam action forces act in a direction tending to separate the rack and pinion elements and may become very high particularly during retraction. Thus, the resulting reaction forces, with prior constructions, have created a rocking couple or moment about a longitudinal axis tending to distort and damage the structure. Problems have resulted due to such stresses under the severe loadings imposed by conditions encountered with long travel blowers. In addition it has been necessary to employ very heavy construction to withstand such forces. An important object of the present invention is to provide improved supporting and guiding means wherein the vertical forces resulting from the cam-like reaction between rack and pinion drive means offset each other without imposing stresses which might distort either the track portions or the beam structure.
A related object is to provide such an assembly wherein the loading problems are further relieved by the fact that the structure is lighter in weight in proportion to its strength and stiffness.
A further related object is to provide such a structure which is lower in cost.
Another object of the invention is to provide an improved carriage driving system.
Other objects and advantages will become apparent to persons skilled in the art upon consideration of the present disclosure in its entirety.